glx: Be sure to set an error for ghost contexts

Otherwise the caller is going to return garbage memory for the error
value.
Signed-off-by: Adam Jackson <ajax@redhat.com>

glx/glxcmds.c

@@ -137,8 +137,10 @@ validGlxContext(ClientPtr client, XID id, int access_mode,
                 __GLXcontext ** context, int *err)
 {
     /* no ghost contexts */
-    if (id & SERVER_BIT)
+    if (id & SERVER_BIT) {
+        *err = __glXError(GLXBadContext);
         return FALSE;
+    }
 
     *err = dixLookupResourceByType((void **) context, id,
                                    __glXContextRes, client, access_mode);